If you use Miracle Grow you want the Miracle Grow Organic Potting Soil. Do Not get the potting "mix", it has inert ingredients such as styrofoam that you don't want in it. I use Just Natural Organic Potting Soil, 1cu ft bags, bought at lowes. This is a heavy mix that will leech alot of tannins the first week.
And yes, dirt tanks get capped with a layer of gravel, sand, Eco-complete, or any number of other substrates.